This invention relates to sensor characteristic adjustment circuits for adjusting the characteristics of a semiconductor sensor.
FIG. 9 is a circuit diagram showing a sensor characteristic adjustment circuit for adjusting the output, characteristic of a semiconductor sensor. The output of a semiconductor sensor 1 is coupled to the inverting input terminal of an operational amplifier 2 through a resistor 3. A variable resistor 4 is coupled across the inverting input terminal and the output terminal of the operational amplifier 2. Across a reference voltage source 5 and the ground 6 are coupled, serially a resistor 7 and a variable resistor 8. The positive (non-inverted) input terminal of the operational amplifier 2 is coupled to the junction point between the resistor 7 and the variable resistor 8.
Next, the operation of the circuit of FIG. 9 is described. Let V.sub.S represent the output voltage of the semiconductor sensor 1, V.sub.0 represent the output voltage of the operational amplifier 2, R.sub.3 represent the resistance of the resistor 3, R.sub.4 represent the variable resistance of the variable resistor 4, and V.sub.P represent the input voltage to the positive (non-inverted) input terminal of the operational amplifier 2. Then, the output voltage V.sub.0 of the operational amplifier 2 is given by: EQU V.sub.0 =V.sub.P (1+R.sub.4 /R.sub.3)-(V.sub.S (R4/R3)
Further, let R.sub.8 represent the variable resistance of the variable resistor 8. Then, the input voltage V.sub.P to the positive (non-inverted) input terminal of the operational amplifier 2 is then given by: EQU V.sub.P =V.sub.REF (R.sub.8 .multidot.(R.sub.7 +R.sub.8))
Thus, the output V.sub.0 of the operational amplifier 2 is given by: EQU V.sub.0 =V.sub.REF (R.sub.8 .multidot.(R.sub.7 +R.sub.8))(1+R.sub.4 /R.sub.3)-(V.sub.S (R4/R3)
In the last equation, the first term on the right hand side represents the setting of the offset level of the sensor characteristic adjustment circuit and the second term represents the setting level of the sensitivity thereof.
Thus, when the output voltage of the semiconductor sensor 1 is 0, only the first term remains in the right hand side of the last equation. The first term does not depend on the output voltage V.sub.S of the semiconductor sensor 1 and hence is a constant. The first term is thus the offset of the sensor characteristic adjustment circuit. On the other hand, the second term is proportional to the output voltage V.sub.S of the semiconductor sensor 1 and thus represents the sensitivity of the sensor characteristic adjustment circuit. The semiconductor sensor 1 may exhibit a random sensitivity variation due to production variations, etc. However, the output V.sub.0 of the operational amplifier 2 (namely, the output of the sensor characteristic adjustment circuit) can be adjusted to a predetermined sensitivity level by adjusting the variable resistance R.sub.4.
The sensitivity and the offset of the sensor characteristic adjustment circuit of FIG. 9 can be adjusted as follows. First, resistance of the variable resistor 4 is adjusted to set the sensitivity of the output of the sensor characteristic adjustment circuit. Next, the resistance of the variable resistor 8 is adjusted to set the offset thereof.
By the way, in the case of the sensor characteristic adjustment circuit of FIG. 9, the operational amplifier 2 is operated as an inverting amplifier. Thus, as the output V.sub.S of the semiconductor sensor 1 increases, the output V.sub.0 of the sensor characteristic adjustment circuit decreases. However, the output of the operational amplifier 2 may be coupled to another inverting amplifier, such that the output of the sensor characteristic adjustment circuit has the same polarity as the output V.sub.S of the semiconductor sensor 1.
The conventional sensor characteristic adjustment circuit of FIG. 9, however, has the disadvantage that variable resistors 4 and 8 are necessary for adjusting the characteristic (i.e., the sensitivity and the offset) thereof. This constitutes an obstacle to a higher degree of integration of the semiconductor sensor 1 and the operational amplifier 2 in an integrated circuit (IC).